As society advances, traditional energy sources such as petroleum and coal are decreasing in storage, solar energy is getting more and more attention as an alternative of traditional energy sources. In photovoltaic applications, solar cells are key elements in photoelectric technology for directly converting sunlight into electric energy, and are widely employed in applications from those for universe to home.
The core of a solar cell is P-N junction. Electron-hole pairs are generated when sunlight with energy higher than a semiconductor's band gap energy is incident on the P-N junction of a solar cell. With the electric field generated on the P-N junction, electrons are transported to an N layer, and holes are transported to a P layer at the same time, thereby resulting in a photoelectric effect between the P and N layers. When two terminals of the solar cell are connected to a load or system, electric energy in the form of current is generated.
Solar cells are classified into various types in terms of materials for forming intrinsic layers, i.e., light absorption layers. Generally, silicon solar cells with intrinsic layers made from silicon are the most common type. There are two types of silicon solar cells nowadays: crystal (single-crystalline or polycrystalline) solar cells and film (amorphous or microcrystalline) solar cells. In addition to these two types of solar cells, there are cadmium-telluride or copper-indium-selenium (CIS, CuInSe2) compound film solar cells, III-V materials based solar cells, dye-sensitized solar cells and organic solar cells, etc.
Solar cells with a single-crystalline silicon substrate have significantly higher conversion efficiency compared to other types of solar cells. However, their fatal defect is a high fabricating cost due to use of single-crystalline silicon wafers. While polycrystalline silicon solar cells may be produced with relatively low fabricating cost, however, polycrystalline silicon solar cells and monocrystalline silicon solar cells do not differ significantly since these two types of solar cells are both made from a large amount of raw materials. Therefore, the high price of raw material and complex fabricating process make it difficult to reduce fabricating cost.
As one solution to address the defects of these crystal solar cells, thin film silicon solar cells have obtained much attention because of their very low fabricating cost since they are fabricated by depositing silicon films on glass as the absorption layers. In fact, thin film silicon solar cells may be made 100 times thinner than crystal ones.
Thin film solar cells are fabricated by sequentially performing the following steps: forming a front (back) electrode on a substrate made of glass, forming a semiconductor layer on the front (back) electrode and forming a back (front) electrode on the semiconductor layer.
In prior art solar cell fabricating technology, thick films (on the order of 1 μm) of transparent conductive oxides (TCO) are typically used to make electrodes. Further, a surface of the TCO thick film is textured by an etching process using photolithography, an etching process using anisotropy of a chemical solution or a mechanical etching process to form an uneven surface, thereby trapping a weakly absorbed portion of the solar spectrum. FIG. 1A shows a scanning electron microscope (SEM) image of a solar cell which employs this technology and a schematics view of the solar cell. FIG. 1B shows an image of a μc -Si:H film grown on a textured ZnO film.
In FIG. 1A, gray regions in the SEM image indicate highly textured ZnO:B (boron doped zinc oxide) film. The two black regions indicate two PIN junctions (a-Si:H on the top, and μc-Si:H at the bottom). It is clear from FIG. 1A that the surfaces of both PIN junction films are uneven, resulting in a rough and textured surface of the lower layer. This can be more clearly seen in FIG. 1B. This texture-induced roughness of junction surface has a deleterious influence on junction quality such as a low shunt resistance, a high dark current, a high carrier recombination rate, a low fill factor and a low open circuit voltage and, therefore, lowers the energy conversion efficiency and reliability of the solar cells.
In addition, this light trapping scheme is not efficient in the frequency ranges closely above the band gap energy of the absorber (i.e., near infrared frequencies), causing light in these frequency ranges to be mainly absorbed by the absorber. This forces the use of thick absorbers, which further deteriorates their performance, in particular in the case that the absorption layer is a-Si:H junction, due to the optical radiation induced performance deterioration (SWE, Staebler-Wronski Effect), the junction performance may be further deteriorated. Furthermore, this may result in a long deposition time, in particular in the case that the absorption layer is a μc-Si:H junction.
In addition, deposition of textured TCO films also involves a high temperature and a high cost.
It could therefore be helpful to provide an electrode and a manufacturing method thereof which need not use or use little transparent conductive oxide films while manufacturing the electrode, and doesn't need a piling process, thereby preventing junction quality from deteriorating due to the piled conductive oxide thick film. In addition, it could be helpful to provide for the use of the planar electrode as a window superstrate, thereby allowing for a high quality planar PV junction to be deposited over it.
Furthermore, it could be helpful to provide another electrode and a fabricating method thereof, with which a texturing process for back electrodes is not required when manufacturing solar cells.